Analysis of the corrosion causes of flue gas exhaust fans in petrochemical industries

Thread Content

This post was last edited by Wang Wei2 on 2023-10-8 at 11:01. Analysis of the causes of corrosion in exhaust fans used in the petrochemical industry 1. Overview: The fuels used in the heating furnaces of the petrochemical industry are gas and heavy oil. Both fuels contain sulfur or hydrogen sulfide. The inlet to the exhaust fan is flue gas coming from the outlet of the heat pipe heater; the designed temperature is 160°C, while the actual operating temperature is around 130°C. Due to the high concentration of sulfur dioxide gas in the flue gas, it causes severe corrosion to the casing and impeller of the induced draft fan. Within less than half a year of use, pitting and significant thinning occurred in the casing, and it became unusable within less than a year. 2. Analysis of corrosion causes: The inducer fan casing is made of carbon steel and is of a volute shape. The flue gas contains N2, O2, H2S, HCl, and CO2. Based on the analysis of the corrosive scale, it contains elements such as Fe 46.75%, Al 16.13%, S 15.79%, Si 15.63%, and Ca 4.04%. This scale has a pH value of 1–2, meaning it is a strong acid, and it is soluble in water. As a result, it can easily cause dew point corrosion on the metal surface of carbon steel. The reasons for this corrosion are as follows: (1) Formation of corrosive agents. Flue gas dew point corrosion occurs when the fuel or gas used in heating furnaces contains sulfur. When such sulfur-containing fuels burn, the sulfur compounds decompose to produce gaseous sulfur or sulfur dioxide, according to the following reactions: H2S + 3/2O2 = SO2 + H2O; 3H2S + 3/2O2 = 3/2S2 + 3H2O. Due to the presence of excess oxygen in the combustion chamber, some of the sulfur dioxide reacts further with oxygen to form sulfur trioxide, as shown in the following reaction: 2SO2 + O2 = 2SO3. In high-temperature flue gases, sulfur trioxide does not corrode metals. However, when the flue gas temperature drops below 400°C, sulfur trioxide reacts with water vapor to form dilute sulfuric acid, as per the reaction: SO3 + H2O = H2SO4. When this dilute sulfuric acid condenses on the metal surface, low-temperature sulfuric acid corrosion occurs. At the same time, the liquid sulfur that condenses on the low-temperature heating surfaces combines with gaseous sulfur and dust present in the flue gases to form hard-to-remove pasty deposits, which increase the thermal resistance and lower the surface temperature of the shell. This further promotes the formation of condensate, and in this cycle, more and more deposits accumulate, leading to electrochemical corrosion beneath those deposits. ⑵ Corrosion of metal surfaces: The large amount of acidic substances present in flue gases leads to an \"acidic recycling cycle\" on the metal surfaces. The rusting of carbon steel in moist air (flue gas) containing sulfur dioxide is considered to be an effect of the \"acid recycling cycle\". According to this concept, sulfur dioxide is first adsorbed on the metal surface, and ferrous sulfate is formed from sulfur dioxide, iron, and oxygen. Then, ferrous sulfate hydrolyzes to form oxides and free sulfuric acid. Sulfuric acid accelerates the corrosion of iron, and the resulting fresh ferrous sulfate is then hydrolyzed to produce free acid, in a continuous cycle like this. The reaction processes are as follows: Fe + SO2 + O2 = FeSO4; 4FeSO4 + O2 + 6H2O = 4FeOOH + 4H2SO4; 4H2SO4 + 4Fe + 2O2 = 4FeSO4 + 4H2O. The presence of sulfur dioxide accelerates the corrosion of carbon steel. 3. Protective measures: This condition can be addressed by applying a polymer coating. The materials are required to have strong impermeability, high corrosion resistance, good anti-scaling properties, excellent temperature resistance, and good water resistance.